The invention relates to a method of and an apparatus for forming a reinforcing cage which is to be embedded, as a structural member, in a prestressed concrete pile.
A reinforcing cage for such prestressed concrete pile may be formed by an arrangement including a stationary cylindrical electrode of a given diameter, a welding electrode disposed for angular movement around the outer periphery of the stationary electrode while maintaining a small clearance therebetween, and a temper electrode which is arranged in the similar manner as the welding electrode. The arrangement is operated so that a plurality of main members or steel bars are brought into surrounding relationship with the peripheral surface of the stationary electrode while maintaining a mutually parallel relationship, and an auxiliary member is sequentially wound, in a helical form, around the outer periphery of the set of main members, with the crossings between the main and the auxiliary members being welded and tempered by an electrical resistance heating which occurs upon passing an electrical current through the stationary electrode on one hand and the welding electrode and the temper electrode, respectively, on the other, with the crossings interposed therebetween. If the welds or crossings are not positively tempered, a degradation in the mechanical properties occur in the main members in the region of the crossings. In the event that there is a high likelihood that such degradation may occur, the welding operation had better be avoided. In recognition of this, the invention provides a method of and an apparatus for forming a reinforcing cage in which during the initial phase of energization or passing the welding current, a detection is made whether the crossings are properly energized to enable a positive welding. If it is determined that the energizing condition is such as to enable a positive welding operation, the energization is continued to permit the welding current of a controlled magnitude to be supplied. Otherwise, the energization is interrupted to avoid the welding operation. The fact that the crossings between the main and the auxiliary members are positively welded is stored, thereby allowing a subsequent tempering operation by passing a tempering current of a controlled magnitude through the crossings which have already been welded in a positive manner. In this manner, a positive tempering operation is assured while avoiding the application of the tempering current through the crossings which have not been welded. The reinforcing cage is thus formed in a sequential manner while avoiding any degradation in the strength of the main members in the region of the welds or crossings.
According to another aspect of the invention, there is provided an apparatus for forming a reinforcing cage in which upon starting the apparatus, the speed of rotation of a rotary electrode is synchronized with the speed of winding an auxiliary member around a circle of main members. Guide bars are disposed on the outer periphery of the stationary electrode so that the force by the rotary electrode is properly applied to the crossings of the main and the auxiliary members in a radial direction of the reinforcing cage, thus assuring the formation of a properly shaped reinforcing cage. The spatial separation between conductors which are connected to the stationary and the rotary electrode, respectively, is reduced to minimize the welding voltage. The conductor connected to the stationary electrode is shaped in a special configuration such that the values of voltage and current supplied to the welds or crossings be uniform as viewed circumferentially of the reinforcing cage, thus assuring a homegeneous welding result.
FIG. 1 illustrates several forms of prestressed concrete piles. By way of example, FIG. 1(a) shows a bottom pile P.sub.1 carrying a conical shoe Sh on its free end and a planar end plate Cap on its upper end. FIG. 1(b) shows a middle or a top pile P.sub.2 carrying planar end plates Cap on its opposite ends for joined use with the bottom pile P.sub.1. Such pile represents a hollow cylindrical concrete body having a length on the order of 7 to 15 meters and a diameter on the order of 300 to 600 milimeters. A reinforcing cage C as shown in FIG. 1(c) is embedded in the annular concrete body. The purpose of the reinforcing cage C is to prevent the occurrence of cracks in the prestressed concrete pile (hereafter referred to as "PC pile") and to improve the rigidity and the bending strength. It includes a plurality of main members 1 in the form of high strength steel bars designed for prestressed concrete use (hereafter referred to as "PC steel bars") having a diameter on the order of 7.4 to 13 milimeters and arranged at a suitable spacing along a circle of a given diameter to define a set 10. To maintain such layout, the cage also includes an auxiliary member 2 in the form of a wire of ordinary iron or soft steel having a diameter from 3.2 to 6 milimeters (JIS 3532), which is wound around the set of main members 1 in a helical configuration, with the crossings of the main members 1 and the auxiliary member 2 being fixedly connected together.
The PC pile P is formed by a process including the steps of disposing the reinforcing cage C in a form or mold while simultaneously tensioning the main members 1, which are the principal structural elements of the set 10, with a tension which is as large as 70% of the tensile strength of the main members 1 (for example, with a tension of 87.5 kg.f/mm.sup.2 when the main members have a tensile strength of 125 kg.f/mm.sup.2), casting fluid concrete into the form, rotating the form about the axis thereof to mold concrete into a hollow cylindrical configuration by utilizing the centrifugal force which results from such rotation, steam curing the assembly to obtain a given strength of the concrete, and releasing the tension applied to the main members 1, thus introducing a prestress into the concrete.
To secure the crossings of the main members 1 and the reinforcing member 2 in forming the reinforcing cage C, the crossings may be connected together by tying with a thin wire, by clamping the both members with clips, or by welding through direct energization of the crossings to produce a resistance heating. Of these techniques, the welding technique through the direct energization is most popular in view of its high productivity.
As mentioned previously, when forming the reinforcing cage C by the welding technique, it is necessary to temper the welds inasmuch as tension of great magnitude is applied to the main members during the time the pile is manufactured and the PC pile P, in which the prestress is applied by embodying the cage C therein, is later driven into earth by a pile driver which applies an impact of great magnitude thereto. The tempering operation is necessary because if the crossings of the main members 1 and the auxiliary member 2 are merely welded without any subsequent treatment, the main members 1 in the region of the welds are hardened to an abnormally high hardness and become brittle as a result of the rapid heating and cooling or quenching during the welding process, and fine cracks may be produced therein as a result of the tension applied to the main bars during the manufacturing of the pile, the impact applied when driving the pile or shocks occurring when the pile is inadvertently dropped during its transportation, which cracks may cause a fracture of the main members 1. Accordingly, it is absolutely necessary that the welds by tempered to restore the inherent strength of the main members 1.
FIGS. 2(a) and 2(b) show a conventional arrangement which is used to weld the crossings of the main members and the auxiliary member and to temper the welds in order to form the reinforcing cage.
FIG. 2 shows a cylindrical stationary electrode 3 of a given diameter, around which a welding rotary electrode and a tempering rotary electrode 4, 5 are disposed in opposing relationship, for example, with an angular spacing of 180.degree. therebetween. The stationary electrode 3 is fixedly mounted on the free end of a cylindrical conductor 31 which is supported in a cantilever fashion by a standard 32 which is electrically insulated. Both the welding and the tempering rotary electrode 4, 5 are mounted on arm members 61 which are attached to the end face of a rotary drum 6 located adjacent to the stationary electrode, the drum 6 having a diameter greater than that of the stationary electrode 3 and disposed in concentric relationship with the conductor 31. As shown, urging members such as coiled compression springs 62 are interposed between the arm members 61 and the electrodes 4, 5. The drum 6 is rotatable at a given speed in a direction indicated by an arrow, by means of a drive unit and a transmission mechanism, both of which are not shown. All of the electrodes 3, 4 and 5 are fed from a welding power source 7, across which the primary side of a welding transformer 71 is connected. The stationary electrode 3 is connected to the secondary side of the transformer through a connecting conductor 72. An annular collector ring 73 is secured to the end face of the drum 6 which is located adjacent to the standard 32, and is engaged by a feeding brush 74 connected to the secondary side of the transformer. A pair of strip conductors 41, 51 are secured to the inner surface of the drum 6 and extend axially therealong and have their one end connected to the collector ring 73 and their other end connected through conductive braids 42, 52 to the rotary electrodes 4, 5, respectively. An annular spool 8 is disposed at a given spacing to the left, as viewed in FIG. 2(a), of the stationary electrode 3, and has a pair of end flanges 81, between which a number of turns of the auxiliary member 2 is received. A drive roll 82 is driven for rotation by a drive unit, not shown, and the resulting rotation is transmitted to one of flanges 81 of the spool, whereby the latter is rotatable in the rewind direction of the auxiliary member 2 which is in the same direction as the direction of rotation of the drum 6. A plurality of guide rolls 83 are disposed along a path of movement of the auxiliary member 2 from the spool 8 to the vicinity of the stationary electrode 3, and are carried by a support arm, not shown, which is fixedly mounted on the drum 6. In this manner, the auxiliary member 2 is paid off the spool 8 to be supplied to a point around the periphery of the stationary electrode 3.
The plurality of main members 1 are inserted into the annular space defined by the outer periphery of the conductor 31 and the inner periphery of the drum 6, from right as viewed in FIG. 2(a). These main members have buttonheads 11 shown in FIG. 1(c) which are retained in position by a locking member 9. In this manner, the main members 1 extend axially in parallel realtionship with each other on a circle disposed around the stationary electrode 3 with a given spacing therebetween. By moving the locking member 9 to the left, as viewed in FIG. 2(a), at a given rate, the main members simultaneously move over the stationary electrode 3. Under this condition, the spool 8 may be rotated at a given rate to pay off the auxiliary member 2, which is then supplied to a point around the set 10, thus allowing the auxiliary member 2 to be wound helically around the set 10. At the same time, the rotary drum 6 is driven for rotation in the given direction and the welding power supply turned on, whereby the crossing of the main member 1 and the auxiliary member 2 is pressed by the welding electrode 4, which urges the crossing toward the stationary electrode 3 under the force of the member 62, as shown in FIG. 2(b ). The welding current is passed for a given cycle under this condition, producing a resistance heating of the crossing, which is therefore welded together. After the drum 6 has rotated through 180.degree. in the direction of the arrow, the same crossing which is welded is pressed by the tempering electrode 5, which urges the crossing toward the stationary electrode 3 under the resilience of the member 62. The tempering current is passed for a given cycle under this condition, thus producing a resistance heating effect of the crossing to achieve a tempering of the crossing. The cyclical application of the welding current and the tempering current is achieved by providing a proximity switch which produces an output whenever the welding rotary electrode 4 has rotated through a predetermined angle which depends on the number of main members. In response to an output from the proximity switch, the application of either current is initiated and continues over a given time interval, by applying a voltage across the stationary electrode 3 and either rotary electrode 4 or 5.
As the set 10 continues to move to the left, the auxiliary member 2 is sequentially wound around the set, with the crossings of the main members 1 and the auxiliary member 2 being successively welded and tempered, thus forming the reinforcing cage C as shown in FIG. 1(c).
FIG. 3(a) shows a cross section of the main member 1 in the region of the welded crossing. Assuming that an ideal welding operation and tempering operation have taken place in the crossing of the main member 1 and the auxiliary member 2, the resulting hardness will be as shown graphically in FIG. 3(b). Specifically, FIG. 3(b) shows measured values of the hardness taken along a line A--A in a region 1a of the cross section of the main member 1 which is affected by the welding process. As shown, the hardness rapidly decreases from a hardness level Th in the surface layer which is abnormally high, for example, MHV610 as compared with the usual hardness level T.sub.0 which may be micro-Vickers hardness MHV480, to a hardness level T.lambda. of MHV350, for example, which is abnormally low as compared with the hardness level T.sub.0 followed by returning to the hardness level T.sub.0 when proceeding from the surface to the center of the main member 1. However, after the tempering operation which follows the initial welding operation, the characteristic hardness curve will be as shown in FIG. 3(c) in which the hardness level Th in the surface layer of the main bar 1 as shown in FIG. 3(b) disappears.
In the conventional arrangement mentioned above, all of the crossings of the main members 1 and the auxiliary member 2 have been cyclically energized to pass the welding current and the tempering current in a "mechanical" manner. As a result, if the electrode which is utilized to pass the welding current through the crossings experiences a poor contact which prevents the flow of a necessary and sufficient welding current to thereby cause an incomplete weld or if a foreign matter or matters are present between the electrode and the crossings resulting in a failure of the welding operation, the "mechanical" process results in passing the tempering current through the same crossings when the tempering rotary electrode has rotated through 180.degree.. Consequently, these crossings are welded to a further degree or welded for the first time rather than achieving a tempering operation, defeating the very purpose intended. The ultimate result is a high likelihood that a reinforcing cage may be manufactured having main members in which local regions Th of high hardness are present.
Considering now the tempering operation, if the magnitude of a tempering current in excessively high, the crossings will be heated to an abnormally high temperature, and are then rapidly cooled from such temperature, resulting in increasing the hardness to a greater value than that attained during a welding operation. Conversely, if the magnitude of a tempering current is excessively low, the crossings will be only heated to a temperature which is insufficient to achieve a desired tempering effect. It is therefore seen that a proper magnitude of tempering current must be chosen in relation to the magnitude of the welding current. In the prior art practice, a value of tempering current has been determined based upon experiences each time the specification of the main members is changed. However, it is difficult to assure a proper tempering operation, disadvantageously causing a difficulty in achieving a uniform quality or causing a reduction in the production efficiency.
In the prior art arrangement, when winding the auxiliary member 2 around the set of main members 10, both the rotary drum 6 and the spool 8 are driven for rotation in a direction to rewind the auxiliary member 2 from the latter at a rate which is predetermined to prevent the auxiliary member 2 from drooping. However, since the spool 8 has a moment of inertia GD.sup.2 which is substantially higher than the moment of inertia of the rotary drum 6 because of the number of turns of the auxiliary member 2 disposed thereon, an increase in the speed of rotation of the spool 8 is significantly lagging with respect to the rotation of the rotary drum 6 which can be immediately increased to a given value upon starting, thus making it difficult to achieve a synchronization between the both speeds when starting. Consequently, a delay is involved in paying off the auxiliary member 2 to prevent the auxiliary member 2 from being wound around the set of main members 10 in a regular helical form. This results in a distortion in the configuration of the reinforcing cage C manufactured, and when the set of main members 10 is tensioned, the auxiliary member 2 may become separated in the region of the welds to cause damage to the main members 1, giving rise to the likelihood that a fracture thereof may be caused.
A distortion of the reinforcing cage C manufactured with a conventional arrangement is also caused by other factors. Specifically, when the welding rotary electrode 4, which is mounted through the force of the member 62 on the arm member 61 which is fixedly mounted on the rotary drum 6, welds the crossing of the main member 1 and the auxiliary member 2, even if a suitable force is applied by the rotary electrode 4, the main member 1 is subject to a torsional stress in the direction of rotation of the rotary electrode 4 which rotates as the rotary drum 6 rotates in a direction by the arrow shown in FIG. 2(a). Hence, if the force with which the rotary electrode 4 is urged has a great magnitude, the rotary electrode 4 will be urged by the force of the member 62 to move closer to the periphery of the stationary electrode 3 during its movement from one of the main members to another, and when it reaches the next main bar 1, it may press against the main member 1, with the auxiliary member 2 interposed therebetween, with an angle of inclination shown in FIG. 4, causing the main member 1 to be displaced in the direction of rotation. This may also cause a distortion of the reinforcing cage C.
If the welding electrode 4 or the tempering electrode 5 contacts the crossing at an angle, the auxiliary member 2 will not follow a circular helical path, but will depict a helix which is polygonal in section corresponding to the number of the main members 1. Thus, the active surface of both electrodes 4, 5 will also depict a locus which has corners, and an intensive sparking may occur to damage the main members 1 during the welding or the tempering operation, rendering the control of the welding operation difficult.
Considering the conductor 31 associated with the stationary electrode 3 and the conductors 41, 51 associated with the welding and tempering electrodes 4, 5 used in the conventional arrangement, it will be appreciated that a flexible use of the arrangement is permitted by choosing an outer diameter of the conductor 31 which is less than the outer diameter of the stationary electrode 3 and choosing an increased inner diameter of the rotary drum 6 while providing stationary electrodes 3 of various diameters so that reinforcing cages having several different diameters may be produced by changing the particular stationary electrode 3 used. Under these circumstances, the separation G between the conductor 31 on the stationary electrode 3 and the conductors 41, 51 associated with the rotary electrodes 4, 5 has an increased value to increase the impedance across the stationary electrode 3 and the rotary electrodes 4, 5. This requires a higher output voltage from the welding transformer 71. This in turn increases the likelihood of occurrence of sparking when welding the crossings of the main members 1 and the auxiliary member 2. In addition, the path length from the stationary feeder brush 74 to the interconnecting conductor 72 through which both the welding and the tempering current must flow changes as the drum 6 rotates, thereby changing the circuit inductance and hence the magnitude of the welding and the tempering current, as viewed in the circumferential direction of the reinforcing cage. This results in a variation in the quality of the welding and the tempering operation, presenting another difficulty in the quality control.